IT was a ‘lenient babysitter’ who inadvertently introduced the young Heidi Thomas to Upstairs Downstairs – then compulsive viewing for millions in early 1970s Britain.

But little could the 11-year-old Garston schoolgirl, allowed a peek at the series when her parents were out, have realised that three decades later she’d be recreating the early 20th century world of the upper class British home.

A lot of water has gone under the bridge in those intervening years, but with the popularity of recent period dramas like Gosford Park and Downton Abbey, it appears the viewing public’s appetite for big house intrigue is as strong as ever.

Now that appetite is being fed with a new version of the classic BBC series, penned by Heidi, which will be screened in three hour –long episodes starting on Boxing Day.

Christmas has become a happy hunting ground for the Liverpool –born writer, with Upstairs Downstairs following last year’s Cranford, and her adaptation of children’s favourite Ballet Shoes in 2007.

And as far as her latest project is concerned, Heidi says she realises the weight of responsibility to the original ITV show, devised by Jean Marsh and Cranford alumnus Dame Eileen Atkins.

Earlier this year she told the ECHO: “Eileen and Jean are the best of friends, they go on holiday together, and I think they had sneakingly always hoped there’d be a revival.

“But they’ve always been very choosy about it and have turned down a number of opportunities in the past, and it’s not that they want loads of money.

“What they do want is to know that it will be done properly and that standards will be maintained. They’re trusting me with something that’s very precious to them and I feel very much the need to justify that faith.”

So what was it that first attracted the young Heidi to the story of the house and its inhabitants all those years ago?

“What struck me most was the masterly depiction of the First World War and its aftermath, all seen through the lens of life at 165 Eaton Place,” she explains.

“It was actually hugely educational, and sparked my passion for social history.

“However, when I re –watched the entire saga as an adult, I was surprised at the risqué nature of many of the storylines. It was all going on in Eaton Place, but I was so innocent that adultery, illegitimacy and homosexuality had all gone completely unnoticed!”

In Heidi’s 2010 Upstairs Downstairs, the time frame has been shifted forwards to 1936 where the storm clouds of another conflict gather ominously in the background.

And the cast of characters has also changed, apart from Jean Marsh’s Rose Buck – now elevated from housemaid to housekeeper.

It must have been fun then to create a new cast of characters to populate the upstairs and downstairs of the London townhouse.

They include new ‘upstairs’ employers, Sir Hallam Holland (Ed Stoppard) and his wife Lady Agnes (Keeley Hawes) who replace the original Bellamy family, with Dame Eileen, who never appeared in the 70s series, playing Maud, dowager Lady Holland.

Heidi describes the process as “an engrossing and rather emotional experience”.

She says: “Like every other fan of the show, I have vivid memories of the Bellamys and their staff, and was aware that there were ghosts to cherish, and dispel.

“Downstairs, there were obvious roles that needed to be filled – a house like 165 needs a butler, a cook, a maid or two, a footman and perhaps a chauffeur. And that, in turn, demands a family upstairs with high standards and plenty of money.

“But I never wanted just to clone the characters that had gone before.

“From the start, it felt right to have a younger couple at the helm of the house, and Sir Hallam's job at the Foreign Office was an obvious way to bring the tumultuous politics of the 30s centre stage.”

“Putting the staff together proved rather challenging,” admits Heidi. “By 1936 servants were much harder to get hold of, and employers had to take what they could get.

“So I felt at liberty to put together a team of rather wonderful juveniles and eccentrics, some of whom would rather be anywhere else!

“And, of course, the great thrill was to bring back the iconic character of Rose Buck. None of this would have been possible without Jean Marsh. Jean IS Rose, and Rose IS Upstairs Downstairs.”

The writer believes the lasting power of Upstairs Downstairs is in its human relationships, both up, down and between stairs (she maintains it’s only partly about class) and the intimacy of its four walls.

Recreating that world in 2010, she says, about preserving “as much of the magic as we could” while tempering it to modern day viewing habits.

“A great deal of the power of the original stemmed from the fact that so much of the drama was sparked by, and played out within, the walls of 165,” she says.

“But in the original series, there was a tendency towards very long scenes that might feel slightly theatrical and slow to a modern audience, so we recalibrated this slightly, while striving to keep the drama detailed and satisfying.

“We also make use of incidental music, which wasn't a feature in the original series, and the sumptuous attention to visual detail is also new.”

Upstairs Downstairs is broadcast on BBC1 at 9pm for three consecutive nights on December 26 –28.